Method of making a valve bag

ABSTRACT

A side weld bag having a patch valve located at one end of the bag. Preferably the bag ends are gusseted. The bag is manufactured by attaching a valve patch so as to cover an opening in the free border of a folded length of film. The edges of the folded length are then sealed together to form a bag having a valve in one end. The bag can be manufactured using a modification of a known bagmaking machine.

United States Patent Worndl 1 Mar. 7, 1972 METHOD OF MAKING A VALVE BAG Inventor;

Canadian Industries Limited, Montreal, Quebec, Canada Jan. 26, 1970 Assignee:

Filed:

Appl. No.: 5,567

Foreign Application Priority Data Feb. 17, 1969 Great Britain ..8,507/69 or KL93I8Y5ZZ2IZ vnr 'wgl r'f'f'lll'f'l"ff"Tl "1'Y'Yfi'i'fff:'lf f l'l ;bfl 4 h2 Field of Search ..93/8 VB, 35 R; 229/625 39!! Wet mnpwntQntatioiuCaqade v [56] References Cited UNITED sTATEs PATENTS 3,280,705 10/1966 Gennerich et al. ..93/8 VB x 3,548,722 12/1970 Jones et al. ..93/8 VB FOREIGN PATENTS OR APPLICATIONS 1,230,299 12/1966 Germany ..93/& v1; 1,016,381 10/1962 Great Britain ..93/s VB Primary Examiner-Hemard Stickney Attorney-Alexander O. McIntosh [5 7] ABSTRACT A side weld bag having a patch valve located at one end of the bag. Preferably the bag ends are gusseted, The bag is manufactured by attaching a valve patch so as to cover an opening in the free border of a folded length of film. The edges of the folded length are then sealed together to form a bag having a valve in one end. The bag can be manufactured using a modification of a known bagmaking machine.

6 Claims, 12 Drawing Figures PATENTEDMAR 7 I972 3.646.856

sum 1 OF 7 INVENTOR John WORNDL AGENT PATENTEDMAR 7 I872 sum 2 [IF 7 We? Mia-.4

AGENT PATENTEHMR 7 I972 SHEET 3 [IF 7 INVENTOI; John WORNDL AGENT PATENTEBMAR 7 I972 SHEET 8 [1F 7 [NI LN I UR John WORNDL am wdkiwm AGENT PATENTEDMAR 7 I972 SHEET 7 0F 7 INVENTOR J'ohn WORNDL muwwfl, ac 11% AGENT METHOD OF MAKING A VALVE BAG This invention relates to a valve bag suitable for manufacture by automatic bagrnaking machines and to a method for manufacturing the same.

Valve bags are in general use for packaging materials such as fertilizers and cement. These bags possess the important commercial advantage of easy filling and closing after filling. However, the presence of the valve in the bag structure introduces problems in bag manufacture and may require manual operation of certain steps in the manufacturing process. There is thus need for a valve bag of simple construction that can be manufactured employing automatic bagmaking machines.

It has now been found that by heat-sealing a patch of plastic film to the border of a folded two ply length of plastic film it is possible to fabricate a plastic bag having a valve positioned in the end wall. Preferably the end wall of the bag is gusseted. The patch can be sealed to the border of the length of folded film employing an attachment to a known bagmaking machine so that the valve afiixture can be integrated into a known bagmaking procedure.

It is therefore a primary object of this invention to provide a valve bag that can be readily manufactured employing automatic bagmaking machines. Additional objects will appear hereinafter.

The valve bag of this invention comprises front and back panels joined along two opposed edges to form a flattened tube, the remaining two opposed edges of the front and back panels being joined to form two bag ends, one of said ends having a patch attached to its inner surface by a pouch-shaped seal, said patch-carrying end having an opening therethrough located within the area of said seal, the said patch and said opening forming a valve. It is convenient to employ an extension of thinner gauge film attached to the free edge of said valve patch. In a preferred form of the invention the bag ends are constituted by gusseted end panels.

The valve bag is manufacture by a method which comprises the steps of:

l. forming a longitudinally folded length of heat-scalable plastic film having facing panels of unequal width, the excess width of the wider panel constituting the valve-carrying end of the completed bag;

2. forming an opening in said end;

3. heat-sealing a patch of plastic film to said end so as to cover said opening, but leaving at least one edge of the patch unsealed, thus forming a valve;

4. heat-sealing together the free edges of the two facing panels longitudinally along a distance equal to the end of the completed bag and including the sealed patch, thus forming a flattened tube having a valve in one edge;

5. severing the two facing panels traversely at positions cor responding to the two side seals of a completed bag; and

6. heat-sealing the two facing panels adjacent to the two transverse severed edges, to form a bag.

The method of manufacturing the preferred form of the bag wherein the ends are constituted by gusseted panels comprises the steps of:

l. forming a longitudinally folded length of heat-scalable plastic film having facing panels of unequal width, the excess width of the wider panel constituting the valve carrying end panel of the completed bag;

2. forming an opening in said end panel;

3. heat-sealing a patch of plastic film to said end panel so as to cover said opening, but leaving at least one edge of th patch unsealed, thus forming a valve;

4. heat-sealing together the free edges of the two facing panels longitudinally along a distance equal to the end of the completed bag and including the sealed patch, thus forming aflattened tube having a valve in one edge;

5. forming in each edge portion of said flattened tube a gusset having a reentrant angle and two external folds, one of the gusseted portions having the valve therein;

6. heat-sealing the two plies of each external gusset fold at tranversely opposite positions of the tube along lines truncating the corners of the section of tube which corresponds to the completed bag; said lines forming four V- shaped seals with apices at the reentrant angles of the gusset;

7. severing the two plies of film at positions external to the V-shaped seals in the gusset folds;

8. heat-sealing the two facing panels transversely along lines joining the apices of transversely opposite V-shaped gusset seals; and

9. severing the two facing panels transversely along lines external to the two transverse seals, thus forming a single bag.

It is convenient to carry out the method as a repeated sequence employing a bagmaking machine in which a folded length of plastic tube is subjected sequentially to the bag forming steps.

The seal joining the valve patch to the end panel conveniently is U-shaped with the opening in the end panel lying at the interior of the base of the U. The seal joining together the free longitudinal edges of the two facing panels preferably is the lap type.

The invention is illustrated by the accompanying drawings wherein:

FIG. 1 is a front elevation of a valve bag without end gusset;

FIG. 2 is a perspective view of a gusseted valve bag in filled configuration;

FIG. 3 is a perspective view of a gusseted valve bag in partially collapsed configuration;

FIG. 4 is a plan view of a portion of the top of the bag of FIG. 2 showing the structure of the valve;

FIG. 5 is a sectional view along V of FIG. 2;

FIG. 6 is side elevation of a bagrnaking machine suitable for manufacturing the valve bag of this invention;

FIG. 7 is a plan view of the machine of FIG. 6;

FIG. 8 is an enlarged view of part of the machine of FIG. 6;

FIG. 9 is a rear elevational view of a device for automatically attaching the valve panel to the bag wall;

FIG. 10 is a front elevational view of the device of FIG. 9;

FIG. 11 is a side elevational view of the device of FIG. 9; and

FIG. 12 is a diagrammatic representation of the control system of the device of FIG. 9.

Referring to the drawings, the bag structure is shown in FIGS. 1 to 5.

In its simplest form the bag has the form shown in FIG. 1. Front and back panels are joined by side seals 3 and 4. The lower end of the bag is the folded junction of the front and back panels. The top of the bag is formed by the folded over extension of the back panel joined to the front panel by seal 8. In this folded over extension is bag opening 9 and patch 10 sealed to the inner surface of the extension. Also, 9 and I0 constitute the valve of the bag.

In its gusseted form the bag is illustrated in FIGS. 2 to 5. This bag is constituted by front panel I and back panel 2 joined by transverse side seams 3 and 4. The ends of the bag are formed of gusseted bottom panel 5, and gusseted top panel 6. Side extensions of the bottom and top panels are sealed to the front and back panels by V-shaped side seals 7. Top panel 6 is joined to the upper longitudinal edge of the front panel by a lap seal 8.

The valve is formed by slit 9 and valve patch or panel 10. Panel 10 is sealed to the inner surface of top panel 6 by U- shaped seal 11. The valve shown in FIG. 4 has an extension of thinner gauge film 12 sealed to its free extremity by seal 13.

A machine suitable for the manufacture of the valve bag of this invention is illustrated in FIGS. 6, 7 and 8. This is a modification of a machine used for manufacturing plastic bags from lengths of plastic tubing. This type of machine advances intermittently a length of plastic film by increments equivalent to a bag length, sealing and cutting operations being carried 5 out while the film is stationary.

At 14 is shown a length of plastic tube being unwound from a supply reel. The tube passes over freely rotatable rolls l5 and 16 to slitting-folding assembly 21. Assembly 21 slits the upper ply of the plastic tube longitudinally at a position distant from the tube edge by half the width of the bag top. The free edge of the upper ply is then unfolded by assembly 21 to form a folded double panel of film, the lower panel being wider than the upper. The protruding edge of the lower panel provides the film for forming the bag top. The double panel film next passes over freely rotatable rolls l7 and 18 to a valve inserting station, indicated generally at 22. Roll 17, in distinction to rolls 15, 16 and 18, is free to move vertically being mounted in frame 19. The vertically motion of roll 17 keeps the film taut during the intermittent movement of the film as it passes through the several steps of the bagmaking operation.

The two ply length of film may be formed alternatively by folding longitudinally a length of single ply film. When formed by slitting tubular film the slitting operation can be carried out as a preliminary step incorporated in the bagmaking machine, as described above, or as an additional step associated with film extrusion.

The length of film indicated generally by 20 next has a valve panel attached at valve inserting station 22. Here knife device 23, acting when the film is stationary, pierces the lower panel to form opening 9 of the valve. The valve panel 10 is formed of two sections as shown in FIG. 4. The material for the valve panel is drawn from supply reels 24 and 25. The material of reel 25 is thinner gauge than that of reel 24, serving to form the thinner extension of the valve panel. The two lengths of film drawn from 24 and 25 are positioned so that their adjacent edges overlap. The overlapped edges pass through heatsealing device 26 which during a period when the film is stationary seals the two strips of film together longitudinally. The compound strip of film next passes between metering rollers 27 and 28. A predetermined size of valve panel is thus advanced over the border of the lower panel of film 20. A cutoff knife cuts off the valve panel and an air-driven clamp holds it in contact with the lower panel border. U-shaped heat-sealer shown generally at 29 then seals the valve panel to the lower panel border at a position of opening 9 thus forming a valve. The sealing of the valve panel of course takes place while the main film web is stationary.

The two-ply length of film with valve panels sealed in place next passes around former plate 30. Former plate 30 is fastened at its upstream end by anchor plate 31 to the frame of the bagmaking machine. The downstream end of the former plate floats on the lower roll of roll assembly 32. The folded length of two-ply film fits around former plate 30 so that the fold rides along the edge of the plate. The advancing film next passes by folding device 33 which folds the edge of the lower panel over the edge of the upper panel. This device consists of two parts, a wedge 34 which elevates the border of the lower panel and a U-shaped guide 35 which turns the raised border over the edge of the upper panel. The edges of the lower and upper panels are now in a position for heat-sealing to form a flattened tube.

The flattened tubelike film with edges overlapping and held in shape by former plate 30 passes to sealer assembly 36. The sealer assembly comprises an upper sealer bar 37 and a matching lower anvil bar 38 carried at the corresponding extremities of two matching lever linkages. The opposite extremities of each lever linkage is connected to an air cylinder and piston 39 to provide power to press the sealer bar against the anvil bar. The edge of former plate 30 passes between sealer bar 37 and anvil bar 38 in such configuration that the overlapping edges of the upper and lower panels lie beneath sealer bar 37. When air is applied to air cylinder 39 sealer bar 37 and anvil 38 compress the overlapping edges of the upper and lower panels between the former plate and sealer bar forming a heat-seal. Upper and lower panels thus have been sealed to form a flattened tube. This seal is seal 8 of the finished bag.

The flattened tube passes through roll assembly 32 to the gusset-forming station. The gusset former comprises four sets of double discs 40 and 41 carried inside the plastic tube by linkage 42 which is fastened to the extremity of former plate 30. Discs 40 and 41 expand the tube into a rectangular cross section. External gusset forming discs 43 fit between the edges of internal double discs 41 and cooperate therewith to form a gusset fold in each edge of the tube. The gusseted tube next passes between roll assembly 44 which compresses the gusseted edges of the tube. The flattened gusseted tube next passes between heated clamp plates 45 which during periods when the film is stationary compress together the gusseted edges of the tube in order to set the gusset folds. The clamp plates are actuated by air cylinder assemblies similar to those of sealer 36.

The gusseted tube next passes to driven draw roll assembly 46. The rolls of this assembly are driven intermittently by a controller to advance the tube in increments equivalent to one bag width. Preceding rolls l5, l6, 17, 18, 32 and 44, in distinction, turn freely being moved only by the moving film in contact with their surfaces.

The flattened gusseted tube next passes between two sets of undriven rolls 47 and 48 to gusset seal forming station 49. At this station V-shaped seals corresponding to seals 7 of the finished bag are made. The sealing elements comprise upper and lower Vshaped sealer-cutter bars 50 arranged in two pairs at each side of the gusseted tube. The sealer-cutter bars of each pair are carried by a lever linkage actuated by an air cylinder assembly. The anvil against which the V-shaped sealer bars compress the gusset fold is a V-shaped plate fitting into the internal angle of the gusset. This plate serves as anvil for both upper and lower sealer-cutter bars. Located within the V-area of the sealer-cutter bars is an air-cylinder-actuated plunger which is controlled to push out the triangular shaped four-ply piece of film external to the V-seal.

The tube next passes to gusset turn over guide 51. This is a device which folds the upper gusset of each side of the tube against the middle portion of the tube. This folding facilitates the passage of the gusseted tube between succeeding rolls.

The tube next passes over roll 52 to the photoelectric scan assembly 53. This assembly comprises a set of three rolls which guide the film under a photoelectric cell. The cell senses the presence of fiduciary marks .printed on the surface of the tube at intervals of one bag width. The signal from the photoelectric cell actuates the controller of the bagmaking machine which in turn controls the several elements of the machine.

The tube next passes to a transverse sealer-cutter unit 54. This is a standard type of mechanism which is controlled to form two parallel transverse seals joining the apices of transversely adjacent V-shaped gusset seals and to sever the two plies of film between these two transverse seals. Unit 54 thus forms the side seals 3 and 4 of two consecutively produced bags and separates the two bags. Unit 54 normally will comprise a set of driven rolls, driven in synchronism with rolls 46, two sets of transverse sealer bars and a transverse severing knife.

The finished bag is ejected from unit 54 onto the moving belt of table 55.

The valve forming device is illustrated in more detail in FIGS. 9-12. This device provides cooperating means for automatically cutting the valve opening in the border of the main length of film, metering out a predetermined length of valve panel film, cutting off this length, clamping the cut off valve panel in correct position over a valve opening and heat-sealing the panel in place. The device uses a control system integrated with the controller of the bagmaking machine with which it is operated.

The operating elements of the device are supported by a frame comprising base plate 56, vertical plates 57, 58 and 59 and longitudinal plate 60. Vertical plates 57, 58 and 59 are recessed at midheight and a table plate 61 is fastened to the lower part of the recess.

Vertical plates 58 and 59 carry two valve panel feed rollers 27 and 28. Upper feed roller 27 rotates freely on shaft 62 which is mounted in bearings 63 and 64. Bearings 63 and 64 are free to move in a vertical sense being carried slideably in rectangular recesses 65 and 66 in vertical plates 58 and 59. Each end of shaft 62 fits into a bore in the end of pistons 67 and 68 of air cylinder assemblies 69 and 70. The cylinders of these assemblies are attached to vertical plates 58 and 59. Manual controls of the air supply to these two air cylinders permits the raising and lowering of upper feed roller 27 to permit insertion of a web of valve panel film in the device during starting up of the bagmaking machine.

Lower feed roller 28 is fixed to its shaft 71 which rotates in bearings in vertical plates 58 and 59. One end of shaft 71 is connected through air clutch 72 to pinion 73. Pinion 73 is in mesh with rack 74 which is attached to the piston of air cylinder assembly 75. This air cylinder assembly is carried rigidly by plate 76 affixed to vertical plate 59. Plate 76 also carries rack bearing block 77 and piston displacement limit stop 78. A rod-shaped extension 79 of rack 74 passes through a bore in stop 78. Double nuts 80 and 81 fitting on threaded portions of 79 can be adjusted to define the displacement of the piston of air cylinder assembly 75. This displacement corresponds to the degree of rotation of feed roller 28. The other end of shaft 71 is attached to one element of overrunning clutch 82. The other element of this clutch is affixed to vertical plate 58. Clutch 82 is fitted so that feed roller 28 can turn in one sense only, this being adapted to feed valve panels into the device.

Feed rollers 27 and 28 have matching annular grooves. Feed plate 83 carries slots through which the feed roller annular surfaces project. Also, 84 is a hand wheel for manual adjustment of the feed rollers.

The knife assembly for forming the slit of the valve is shown generally at 23. This comprises a frame 85 which carries air cylinder assembly 86 with its attached slitting knife 87 and slotted film support 88. The film support is located in a cutout area of table plate 61. Knife support plate 89 is fixed to the end of the piston of air cylinder assembly 86. This support plate carries a vertical pin 90 which slides in a bore located in frame extension 91. This pin acts to prevent the knife from turning about the piston axis. Also, 92 is a knife guard which covers the knife edge until the film is contacted. Frame 85 is movable along the direction of the bag top being attached to plate 60 by a retaining screw 93 fitting in a slot 94 in the plate.

Valve panel cutoff knife is shown at 95. This is protected by a vertically movable guard 96. Knife 95 is carried by crosshead plate 97. Bores in each extremity of 97 slide vertically about rods 98 and 99. Rods 98 and 99 are attached to plate 100 which is also fitted with slot 101 into which knife 95 enters at the end of its cutting stroke. 102 is a plate which shields the knife assembly from the sealer assembly. Midway of the upper part of crosshead plate 97 is cut a slot 102' for receiving the matching T-shaped element 103 carried by the piston of air cylinder assembly 104. Cylinder assembly 104 is attached to horizontal frame plate 60 and provides the power for lowering and raising the cutoff knife 95.

The valve panel sealer assembly is shown generally at 29. This comprises a U-shaped sealer bar 105 attached to plate 106. Plate 106 is affixed to end member 107 of the piston of air cylinder assembly 108. Air cylinder assembly 108 is carried by vertical frame plate 60 and provides the power for lowering and raising sealer bar 105. To the base of air cylinder assembly 108 is attached a plate 109, through a bore in which slides rod 110 attached to sealer bar carrying plate 106. Rod 110 serves to prevent sealer bar 105 from turning about the axis of the air cylinder piston.

Plate 109 also carries clamp air cylinder assembly 1 10. The piston of this assembly passes through a bushing 112 in plate 106 and at its lower end carries clamp plate 113. This clamp serves to clamp the valve panel against the top panel of the bag during the sealing of the valve to the bag.

The sealer bar and clamp assembly are attached to plate 60 by hinge 114. This permits swinging out the sealer bar and clamp to adjust the film web.

At 115 is shown a tongue-shaped guide plate which holds the film web (which serves to form the top panel of the bag) against table plate 61. Plate 115 is shaped so that it fits within the U-shaped confines of the valve panel seal 11. Plate 115 is interposed between the top panel of the bag and the valve panel 10 when the valve panel is clamped in position by clamp plate 113. Plate 115 is attached to the inner edge of plate 61 at a position in advance of the valve panel sealing unit.

At 121 is shown a mechanically operated air valve which, as part of the pneumatic control system, triggers the return stroke of cylinder 75 when contacted by nuts 80. At 128 is shown a mechanically operated electrical switch which when the sealer bar assembly is lowered triggers an electric timer which controls the dwell time of the sealer bar.

The pneumatic circuit for operating the valve forming device is illustrated in FIG. 12. The operating sequence is as follows.

An electrical signal provided by a cam operated switch on the drive unit of draw roll 46 passes by conductor 116 to solenoid operated air valve 117. Air valve 117 is actuated supplying pilot air to air controlled air clutch valve 118 and feed roller cylinder valves 119 and 120. Valve 118 operates, closing air clutch 72, thus connecting air cylinder 75 to feed roller shaft 71. At the same time valve 119 advances the piston of air cylinder assembly 75 thus rotating the feed rollers. At the end of the piston stroke air valve 121 is mechanically actuated to feed pilot air to valves 118, 119 and 120 to reverse their positions opening air clutch 72 and retracting the piston of air cylinder assembly 75. The feed rollers remain stationary on the return stroke of the piston because of the action of the overrunning clutch 82. At the same time valve 121 supplies pilot air tocutoff actuating valve 122 and clamp valve 123 thus lowering the cutoff knife 95 and valve panel clamp 113. The valve panel material has now been fed to proper length, cut off and clamped in position against guide plate 115 lying over the bag top panel.

An electrical signal from the bagmaking machine controller carried by electrical conductor 124 next actuates solenoid controlled air valve 125. Valve 125 feeds pilot air to cutoff valve 122, slit knife valve 126 and sealer bar valve 127. These valves feed air to the respective air cylinders to lower the slit knife, raise the cutoff knife and lower the sealer bar assembly. As the sealer bar assembly is lowered switch 128 (FIG. 11) whose arm has been in contact with plate 109 of the sealer assembly, actuates an electrical timer 135. After a predetermined sealing interval has passed a control signal passes from timer 135 along conductor 129 to cause valve 125 to return to its original position. This feeds pilot air to slit knife valve 126 and sealer bar valve 127, causing the slit knife and sealer bar assembly to rise. Exhaust air from sealer bar cylinder 108 is employed as pilot air passing by air line 130 to clamp valve 123 to return this valve to its previous position thus causing the clamp 113 to rise, but lagging in time the rising of the sealer bar.

Air valve 131 is a manually operated valve which permits manual operation of the valve panel cutoff knife.

Air valve 132 is a manually operated valve that serves to prevent the feed roller piston from retracting so that adjustment of the length of stroke of rack 74 can be made.

Air lines 133 and 134 connected to valve 125 serve to provide pilot air for sealer unit 26 which is used for heat-sealing two plastic strips to form a two component valve panel.

Another manually operated air valve, not shown, operates upper feed roller air cylinders 69 and 70 permitting manual control of this roller during adjustment of the device.

The valve bags of the present invention may be formed from any of the many heat-sealable films available such as polyethylene or polypropylene.

The method of the present invention has the advantage that it permits the manufacture of valve bags in a continuous manner from a folded web of plastic film, the valve being inserted automatically in the border of the web. The method also has the advantage that it can be carried out using a modification of known bagmaking machines.

I claim:

1. A method for the manufacture of valve bags having side seams comprising the steps of:

l. forming a longitudinally folded length of heat-sealable plastic film having two free edges, said folded length constituting two facing panels of unequal width, the excess width of the wider panel relative to the narrower panel constituting the valve-carrying end panel of the completed bag;

2. forming an opening in said end panel;

3. heat-sealing a patch of plastic film to said end panel so as to cover said opening, but leaving at least one edge of the patch unsealed, thus forming a valve;

4. folding said end panel longitudinally so that its free edge lies over the free edge of the narrower panel;

5. heat-sealing together the free edges of the end panel and the narrower panel longitudinally along a distance equal to the end of the completed bag and including the sealed patch, thus forming a flattened tube having a valve in one edge;

6. heat-sealing the two facing panels transversely at positions corresponding to the two side seams of a completed bag; and

7. severing the two facing panels at positions external to the two transverse heat-seals, to form a bag.

2. A method as claimed in claim 1 wherein subsequent to Step (5) and prior to Step (6) therein the flattened tube hav- LII ing a valve in one edge is subjected to additional operations with view to forming gusseted end panels comprising the steps of:

A. forming in each edge portion of said flattened tube a gusset having a reentrant angle and two external folds, one of the gusseted portions having the valve therein;

B. heat-sealing together the film of each external gusset fold along truncating lines at transversely opposite positions of the tube corresponding to the comers of the completed bag, said truncating lines forming four V-shaped seals with apices in the reentrant angles of the gusset at positions corresponding to the side seams of the completed bag; and

C. severing the film of the external gusset folds at positions external to the V-shaped seals in said gusset folds.

3. A method as claimed in claim 1 wherein the valve bags are manufactured in repeated sequence, the transverse severing Step (6) serving to separate two sequentially manufactured bags.

4. A method as claimed in claim 1 wherein the forming of 5. A method as claimed in claim 1 wherein the steps of the method are coordinated by means of an electrical and pneumatic control system.

6. A method as claimed in claim 1 wherein the heat-scalable plastic film is polyethylene or polypropylene film. 

1. A method for the manufacture of valve bags having side seams comprising the steps of:
 1. forming a longitudinally folded length of heat-sealable plastic film having two free edges, said folded length constituting two facing panels of unequal width, the excess width of the wider panel relative to the narrower panel constituting the valve-carrying end panel of the completed bag;
 2. forming an opening in said end panel;
 3. heat-sealing a patch of plastic film to said end panel so as to cover said opening, but leaving at least one edge of the patch unsealed, thus forming a valve;
 4. folding said end panel longitudinally so that its free edge lies over the free edge of the narrower panel;
 5. heat-sealing together the free edges of the end panel and the narrower panel longitudinally along a distance equal to the end of the completed bag and including the sealed patch, thus forming a flattened tube having a valve in one edge;
 6. heat-sealing the two facing panels transversely at positions corresponding to the two side seams of a completed bag; and
 7. severing the two facing panels at positions external to the two transverse heat-seals, to form a bag.
 2. forming an opening in said end panel;
 2. A method as claimed in claim 1 wherein subsequent to Step (5) and prior to Step (6) therein the flattened tube having a valve in one edge is subjected to additional operations with view to forming gusseted end panels comprising the steps of: A. forming in each edge portion of said flattened tube a gusset having a reentrant angle and two external folds, one of the gusseted portions having the valve therein; B. heat-sealing together the film of each external gusset fold along truncating lines at transversely opposite positions of the tube corresponding to the corners of the completed bag, said truncating lines forming four V-shaped seals with apices in the reentrant angles of the gusset at positions corrEsponding to the side seams of the completed bag; and C. severing the film of the external gusset folds at positions external to the V-shaped seals in said gusset folds.
 3. A method as claimed in claim 1 wherein the valve bags are manufactured in repeated sequence, the transverse severing Step (6) serving to separate two sequentially manufactured bags.
 3. heat-sealing a patch of plastic film to said end panel so as to cover said opening, but leaving at least one edge of the patch unsealed, thus forming a valve;
 4. folding said end panel longitudinally so that its free edge lies over the free edge of the narrower panel;
 4. A method as claimed in claim 1 wherein the forming of the longitudinally folded length of film of Step (1) is carried out independently of the following steps of the method, the length of film being stored on a supply reel from which film is drawn for the carrying out of Step (2) of the method.
 5. A method as claimed in claim 1 wherein the steps of the method are coordinated by means of an electrical and pneumatic control system.
 5. heat-sealing together the free edges of the end panel and the narrower panel longitudinally along a distance equal to the end of the completed bag and including the sealed patch, thus forming a flattened tube having a valve in one edge;
 6. heat-sealing the two facing panels transversely at positions corresponding to the two side seams of a completed bag; and
 6. A method as claimed in claim 1 wherein the heat-sealable plastic film is polyethylene or polypropylene film.
 7. severing the two facing panels at positions external to the two transverse heat-seals, to form a bag. 